This section introduces aspects that may facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In Long-Term Evolution (LTE) system, the uplink (UL) physical channel “physical random access channel (PRACH)” is used to carry Message 1 (MSG1), i.e., preamble sequences for random access. According to 3GPP TS 36.211 V11.0.0 (2012-09), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 11)”, chapter 5.7.2 “Preamble sequence generation”, the random access preambles are generated from Zadoff-Chu sequences with zero correlation zone, generated from one or several root Zadoff-Chu sequences. The network configures the set of preambles sequences the user equipment (UE) is allowed to use.
There are 64 preambles available in each cell. The set of 64 preamble sequences in a cell is found by including all the available cyclic shifts of a root Zadoff-Chu sequence with the logical index RACH_ROOT_SEQUENCE in the order of increasing cyclic shift, where RACH_ROOT_SEQUENCE is broadcasted as part of the System Information. Additional preamble sequences, in case 64 preambles cannot be generated from a single root Zadoff-Chu sequence, are obtained from the root sequences with the consecutive logical indexes until all the 64 sequences are found. The logic root sequence order is cyclic: the logic index is consecutive from 0 to 837. The relation between a logic root sequence index and a physical root sequence index u is given in 3GPP TS 36.211, and each physical root sequence index corresponds to a logical index RACH_ROOT_SEQUENCE.
The uth root Zadoff-Chu sequence is defined by
                    x        u            ⁡              (        n        )              =          e                        -          j                ⁢                              π            ⁢                                                  ⁢            u            ⁢                                                  ⁢                          n              ⁡                              (                                  n                  +                  1                                )                                                          N                          Z              ⁢                                                          ⁢              C                                            ,      0    ≤    n    ≤                  N        ZC            -      1      where NZC is the length of the Zadoff-Chu sequence, and for preamble format 0-3, NZC equals to 839; for preamble format 4, NZC equals to 139.
From the uth root Zadoff-Chu sequence, floor(NZC/NCS) cyclically shifted sequences are obtained by cyclic shifts of NCS, where NCS is the cyclic shift value used for random access preamble generation, which is broadcasted as part of System Information Block 2 (SIB2).
One of the reasons why LTE system uses cyclically shifted Zadoff-Chu sequences for preamble sequence generation is that the cross-correlation between different preambles based on cyclic shifts of the same root Zadoff-Chu sequence is zero at the receiver as long as the cyclic shift NCS used when generating the preambles is larger than the maximum round-trip propagation time in the cell plus the maximum delay spread of the channel. Therefore, due to the ideal cross-correlation property, there is no intra-cell interference from multiple random-access attempts using preambles derived from the same Zadoff-Chu root sequence.
To handle different cell sizes, the cyclic shift NCS is signaled as part of the System Information.
In small cells with short propagation time, a small cyclic shift NCS, can be configured, resulting in a large number of cyclically shifted sequences being generated from each root sequence. When the number of such cyclically shifted sequences generated from same root sequence exceeds 64, i.e. floor(NZC/NCS)≧64, all the 64 preamble sequences can be generated from a same root Zadoff-Chu sequence so that they can be transmitted on a same random access resource without causing any intra-cell interference to each other.
Herein, “a random access resource” refers to a resource block defined in the time domain and the frequency domain and used for transmitting random access signals, which may also be referred to as a random access opportunity, a PRACH resource, a PRACH opportunity, or a PRACH instance. In LTE systems, in frequency domain, one random access resource usually corresponds to 6 consecutive physical resource blocks (PRBs) and in time domain it usually corresponds to a “PRACH window”. Here, the duration of a “PRACH window” usually consists of a length TCP of cyclic prefix, a length TSEQ of a sequence part, and a guard period GP, and depends on preamble format.
However, in large cells with longer propagation time, whose cell radii are larger than 1.5 km, a larger cyclic shift NCS needs to be configured resulting in less cyclically shifted sequences being generated from a same root sequence. When 0<floor(NZC/NCS)<64, in order to generate all the 64 preamble sequences, multiple root Zadoff-Chu sequences must be used in the large cell. Although the larger number of root sequences is not a problem in itself because there are as many as 838 root Zadoff-Chu sequences in all, the zero cross-correlation property can only hold between shifts of the same root sequence.
Thus, from interference perspective, if one random access resource has to be multiplexed to carry multiple preambles generated from different root Zadoff-Chu sequences, the intra-cell interference from multiple random-access attempts will become unavoidable.
Furthermore, from receiver perspective, according to the existing solution, suppose all the 64 preambles are available in the cell according to SIB 2, the receiver of eNB has to simultaneously detect as many as 64 preambles derived from multiple different root Zadoff-Chu sequences on each PRACH instance, which will be hard for the receiver to achieve “zero erroneous detection”.
In a Chinese patent application publication CN101227734A, a method and apparatus for ranking ZC sequences of random access channel is provided. The method comprises establishing mapping relationship between the logical index and the physical index, such that PRACH of a cell phone which utilizes different sequences in a same cell has almost same coverage range. The proposed method needs to change the mapping relationship defined in 3GPP standards.
In another Chinese patent application publication CN101394226A, a random access preamble having multiple Zadoff-Chu sequence is provided for a cellular telephone system. The random access preamble comprises two ZC sequences so as to lessen the Doppler frequency shift effect.
None of the prior art has addressed one or more of the above concerns.